Cytosolic Antibody Research Articles

Multifaceted role of TRIM21 in inflammation.

Tripartite motif (TRIM) family members participate in a variety of cellular activities, such as intracellular signaling, development, cellular death, protein quality control, immunological defense, waste degradation, and the emergence of cancer. These proteins usually act as E3 ubiquitin ligase. The final line of resistance against infectious viruses is a cytosolic ubiquitin ligase and antibody receptor called TRIM containing 21. TRIM21, a protein with a tripartite structure, has been linked to autoimmune erythematosus, Sjogren's disorder, and innate immunity. TRIM21 may either promote the formation of specific cancer-activating proteins, resulting in their proteasomal degradation, or it may do neither, depending on the kind of cancer and cancer-causing trigger. The current research has shown that the antiviral action of TRIM mostly depends on their role as E3-ubiquitin ligases and a significant portion of the TRIM family mediates the transmission of innate immune cell signals and the subsequent production of cytokines. We highlighted the function of TRIM family members in various inflammatory diseases.

The cytosolic antibody receptor TRIM21 is required for effective tau immunotherapy

AbstractBackgroundUsing a variety of cell and animal model systems we demonstrate that the cytosolic antibody receptor and E3 ligase TRIM21 plays a critical role in antibody‐mediated protection against tau pathologyMethodExperiments with organotypic hippocampal slice cultures and in vivo studies in mouse models were conducted to study the role of TRIM21 in tau immunotherapy.ResultWe demonstrate that tau:antibody complexes were internalised to the cytosol of neurons, enabling TRIM21 engagement and thereby protecting against seeded aggregation. Crucially, antibody‐mediated protection against tau pathology was lost in mice lacking TRIM21ConclusionThe cytosolic compartment provides a site of immunotherapeutic protection, which can help in the design of antibody‐based therapies in neurodegenerative disease.

Immunotherapy against tau pathology in mouse models requires cytosolic Fc receptor TRIM21

AbstractBackgroundThe passive transfer of monoclonal antibodies reduces tau pathology in mouse models and is under evaluation as a potential immunotherapeutic strategy against Alzheimer’s disease and other tauopathies. The mechanisms that drive therapeutic protection of anti‐tau antibodies are not well understood.MethodP301S tau transgenic animals were treated with antibodies against pS422 and analysed for tau pathology by biochemical fractionation. Organotypic hippocampal slice cultures were used as a model of tau seeding and neutralisation. AAV serotypes were used to deliver tau‐selective degraders.ResultGenetic deletion of the cytosolic antibody receptor and E3 ubiquitin ligase TRIM21 rendered tau immunotherapy ineffective in animal models. In organotypic slices, expression of TRIM21 was essential for protection against seeded aggregation of tau. Cytosolic antibody‐tau complexes were visible inside neurons in complex with TRIM21 shortly after application of tau assemblies and antibodies to the cell exterior. Nanobody‐based degraders were developed using TRIM21 and found to protect against tau pathology.ConclusionAnti‐tau antibodies can engage extracellular tau seeds and enter neurons as a complex. Following cytosolic entry, TRIM21 engages the antibodies and mediates neutralisation of tau seeding activity. The results reveal a mechanism that can be optimised for future immunotherapeutics. In this direction, AAV‐delivered degraders reveal a route by which tau aggregates may be selectively removed from the brain.

Anti-Neutrophil Cytoplasmic Antibody-Associated Vasculitis: A Glimpse into The Future

Background: A class of uncommon IgG4-related systemic diseases known as vasculitis caused by antineutrophil cytosolic antibodies (ANCA) is distinguished by the enlargement of small to large blood vessels. Objective: This review article offers an in-depth analysis of the most recent developments in ANCA-associated vasculitis, covering a range of topics from pathophysiology and diagnostics to treatment and long-term results. Methods: In this review we investigated the existing work on ANCA associated vasculitis by different sources such as Science Direct, Scopus, Pubmed, Web of Science, Google scholar and SciHub. Results: A number of diseases, including eosinophilic granulomatosis with polyangiitis (EGPA), microscopic polyangiitis (MPA), and granulomatosis with polyangiitis (GPA), can have significant morbidity and mortality if they are not appropriately diagnosed and treated. Understanding the pathophysiology, clinical presentation, and curative options for ANCA-associated vasculitis have advanced significantly during the past ten years. The development of specific immunosuppressive medications has been largely responsible for the remarkable evolution in recent years in the treatment of ANCA-associated vasculitis. Conclusion: This review article has provided a detailed examination of the therapy for AAV, alternatives available, including induction and maintenance regimens, as well as the accompanying advantages and disadvantages. Additionally, the growing importance of biologic drugs like rituximab was examined, emphasizing its potential as supplements or replacements for traditional medical treatments.

Viral hepatitis is associated with increased risk of decompensated cirrhosis or liver failure in patients positive for liver cytosol antibody type 1.

Liver cytosol antibody type 1 (anti-LC1) is reported to be a marker of type 2 autoimmune hepatitis (AIH), a type of autoimmune liver disease (AILD). However, anti-LC1 is not entirely disease-specific, and its clinical value in other hepatic diseases has not been well elucidated. Our study aimed to explore the associations between the diagnoses and outcome of decompensated cirrhosis or liver failure (DC/LF) in patients positive for anti-LC1. A total of 157 patients positive for anti-LC1 were included in our final analysis. DC/LF was defined as the outcome of patients positive for anti-LC1. The risk of DC/LF according to diagnosis was estimated using multivariable Cox proportional hazards models, while stratified Cox regression models were used in the subgroup analyses. The diagnoses of patients positive for anti-LC1 were found to be comprised of various liver disorders. Versus other diagnoses, viral hepatitis was associated with a 2.25-fold increased risk of DC/LF in these patients, independent of sex, age, disease course, treatment and drinking history. Additionally, the associations were more significant by subgroup analysis in male patients, younger patients, non-newly diagnosed patients, patients without treatment and patients without drinking history. Anti-LC1 is not a disease-specific antibody, as it was found in multiple types of hepatic disease. Furthermore, viral hepatitis rather than AILD was associated with an increased risk of DC/LF in patients positive for anti-LC1. These findings emphasize the important role of viral hepatitis in the progression of DC/LF in patients positive for anti-LC1.

Cytosolic antibody receptor TRIM21 is required for effective tau immunotherapy in mouse models.

Aggregates of the protein tau are proposed to drive pathogenesis in neurodegenerative diseases. Tau can be targeted by using passively transferred antibodies (Abs), but the mechanisms of Ab protection are incompletely understood. In this work, we used a variety of cell and animal model systems and showed that the cytosolic Ab receptor and E3 ligase TRIM21 (T21) could play a role in Ab protection against tau pathology. Tau-Ab complexes were internalized to the cytosol of neurons, which enabled T21 engagement and protection against seeded aggregation. Ab-mediated protection against tau pathology was lost in mice that lacked T21. Thus, the cytosolic compartment provides a site of immunotherapeutic protection, which may help in the design of Ab-based therapies in neurodegenerative disease.

TRIM21-dependent target protein ubiquitination mediates cell-free Trim-Away.

Tripartite motif-containing protein 21 (TRIM21) is a cytosolic antibody receptor and E3 ubiquitin ligase that promotes destruction of a broad range of pathogens. TRIM21 also underlies the antibody-dependent protein targeting method Trim-Away. Current evidence suggests that TRIM21 binding to antibodies leads to formation of a self-anchored K63 ubiquitin chain on the N terminus of TRIM21 that triggers the destruction of TRIM21, antibody, and target protein. Here, we report that addition of antibody and TRIM21 to Xenopus egg extracts promotes efficient degradation of endogenous target proteins, establishing cell-free Trim-Away as a powerful tool to interrogate protein function. Chemical methylation of TRIM21 had no effect on target proteolysis, whereas deletion of all lysine residues in targets abolished their ubiquitination and proteasomal degradation. These results demonstrate that target protein, but not TRIM21, polyubiquitination is required for Trim-Away, and they suggest that current models of TRIM21 function should be fundamentally revised.

Targeted protein degradation by Trim-Away using cell resealing coupled with microscopic image-based quantitative analysis.

"Trim-Away" technology enables rapid degradation of endogenous proteins without prior modification of protein-coding genes or mRNAs through delivery of antibodies that target proteins of interest. Although this approach can be readily applied to almost any cytosolic protein, strategies for cytosolic antibody delivery have been limited to microinjection or electroporation, which require skill-dependent operation or specialized equipment. Thus, the development of antibody delivery methods that are convenient, scalable, and preferably do not require detachment of adherent cells is required to extend the versatility of the Trim-Away method. Here, we developed a cell resealing technique optimized for Trim-Away degradation, which uses the pore-forming toxin streptolysin O (SLO) to permeabilize the cell membrane and delivered the antibodies of interest into HEK293T, HeLa, and HK-2 cell lines. We demonstrated the ability of Trim-Away protein degradation using IKKα and mTOR as targets, and we showed the availability of the developed system in antibody screening for the Trim-Away method. Furthermore, we effectively coupled Trim-Away with cyclic immunofluorescence and microscopic image-based analysis, which enables single-cell multiplexed imaging analysis. Taking advantage of this new analysis strategy, we were able to compensate for low signal-to-noise due to cell-to-cell variation, which occurs in the Trim-Away method because of the heterogenous contents of the introduced antibody, target protein, and TRIM21 in individual cells. Therefore, the reported cell resealing technique coupled with microscopic image analysis enables Trim-Away users to elucidate target protein function and the effects of target protein degradation on various cellular functions in a more quantitative and precise manner.

S2969 Distinction Between Mitochondrial Antibody Positive and Negative Primary Biliary Cholangitis: Case Report and Literature Review

Introduction: Anti-mitochondrial antibody-positive primary biliary cholangitis (AMA-pos PBC) is an autoimmune disorder in which monoclonal antibodies are produced against epitopes with in the mitochondrial membranes of biliary epithelial cells, resulting in progressive non-suppurative biliary cholangitis. Up to 5 % of PBC patients lack these auto antibodies, termed as anti-mitochondrial antibody negative (AMA-neg) PBC. This is a somewhat new entity and a variant of AMA-pos PBC but not an overlap syndrome. There have not been good studies describing this phenomenon or associated terminology in the literature. Case Description/Methods: An 87-year-old woman was referred to our clinic after her medical oncologist found elevated isolated levels of serum alkaline phosphatase (714 units/L). She reported fatigue but denied any other symptoms. The physical examination was benign, except for bilateral lower extremity swelling secondary to lymphedema. Her serum alkaline phosphatase level decreased to 413 units/L after an initial dose of prednisone 40 mg daily, and she was maintained on 10 mg daily. Her antinuclear antibody titer was greater than 1:2560 in a centromere pattern. Anti- mitochondrial antibody was not detected. Total IgG level was 871 mg/dL (normal, < 1600 mg/dL), serum anti-smooth muscle antibody was negative, and the hepatitis panel was normal. Computed tomography of the abdomen and pelvis without contrast showed normal liver parenchyma and no acute intra-abdominal pathology. Histopathological examination indicated florid duct lesions. Background parenchyma showed no significant steatosis, and the inflammatory changes were limited primarily to the portal areas. Periodic acid-Schiff staining highlighted the intact hepatic parenchyma and architecture. The patient was diagnosed with AMA-neg PBC and responded well to Urosdeoxycholic acid therapy. (Figure) (Table). Discussion: This case highlights the importance of recognizing AMA-neg PBC as a variant of AMA-pos PBC and being able to differentiate them. Autoimmune cholangitis is a vague and imprecise term that cannot be used in this context. All AMA-negative PBC patients should be tested for other PBC-specific autoantibodies. Although prognosis and bile duct damage and loss are worse in AMA-neg PBC for unknown reasons, treatment remains the same for both.Figure 1.: A: The position of AMA-neg PBC in the spectrum of autoimmune hepatobiliary cholangiopathy PBC, primary biliary cholangitis; ANA, antimitochondrial antibody; IgM, immunoglobulin M; UDCA, ursodeoxycholic acid; AMA-neg, antimitochondrial antibody negative; ANA, antinuclear antibodies; IgG, Immunoglobulin G; ASMA, anti-smooth muscle antibody; anti-LKM-1, anti-liver kidney microsomal antibodies; Anti-LC-1, anti-liver cytosol antibodies; anti-SLA/LP antibodies, anti-soluble liver antigen/liver pancreas antibodies; MRCP, magnetic resonance cholangiogram; ERCP, endoscopic retrograde cholangio-pancreatogram. B and C:Two photomicrographs showing histologic features typical of the florid duct lesion seen in primary biliary cholangitis.. The portal areas show granulomatous inflammation and lymphocytic cholangitis of the bile ducts (pointed yellow arrows) (hematoxylin and eosin stain, 200X original magnification). Table 1. - Diagnostic criteria of AMA-Pos PBC The diagnosis of AMA-pos PBC can be established when two of the criteria listed in Table 1 are met[5] Evidence Criteria 1) Biochemical evidence of cholestasis Elevated alkaline phosphatase levels 2) Serological:AMA-Pos PBCAMA-Neg PBC Mitochondrial antibody positiveMitochondrial antibody negative/Presence of other PBC specific autoantibodies i.e., sp100 or gp210 3) Histopathological Nonsuppurative destructive cholangitis and destruction of interlobular bile ducts.

Type 2 autoimmune hepatitis: Genetic susceptibility.

Two types of autoimmune hepatitis (AIH) are recognized; AIH-1 is characterized by the presence of anti-nuclear and/or anti-smooth muscle autoantibodies, while AIH-2 is associated with the presence of anti-Liver kidney microsome and/or anti-Liver Cytosol antibodies. The autoantigens targeted by AIH-2 autoantibodies are the cytochrome P450 2D6 and Formiminotransferase-cyclodeaminase for anti-LKM1 and anti-LC1 respectively. Both autoantigens are expressed in hepatocytes at higher levels than in any other cell type. Therefore, compared to AIH-1, the autoantigens targeted in AIH-2 are predominantly tissue-specific. Distinct clinical features are specific to AIH-2 compared to AIH-1, including diagnosis in younger patients (mean age 6.6 years), onset as fulminant hepatitis in very young patients (3 years of age or less), higher frequency in children than in adults and is frequently associated with extrahepatic T cell-mediated autoimmune diseases. AIH-2 is also often diagnosed in patients with primary immunodeficiency. AIH-2 is associated with specific HLA class II susceptibility alleles; DQB1*0201 is considered the main determinant of susceptibility while DRB1*07/DRB1*03 is associated with the type of autoantibody present. HLA DQB1*0201 is in strong linkage disequilibrium with both HLA DRB1*03 and DRB1*07. Interestingly, as in humans, MHC and non-MHC genes strongly influence the development of the disease in an animal model of AIH-2. Altogether, these findings suggest that AIH-2 incidence is likely dependent on specific genetic susceptibility factors combined with distinct environmental triggers.

Atypical presentations of inclusion body myositis: Clinical characteristics and long-term outcomes.

Inclusion body myositis (IBM) typically presents with progressive weakness preferentially involving finger flexors and quadriceps. Atypical presentations have been less commonly reported. Here, we aim to describe the clinical characteristics and long-term outcomes of IBM patients with atypical presentations. We retrospectively searched the Mayo Clinic medical records to identify IBM patients with atypical disease onset, seen between 2015 and 2020. We identified 357 IBM patients, of whom 50 (14%) had an atypical presentation. Thirty-eight patients were diagnosed with IBM because they fulfilled one of the European Neuromuscular Center diagnostic categories at a later stage, 10 had all IBM histopathological features, and 2 were diagnosed on the basis of clinical and laboratory data. The most common presentation was dysphagia (50%), followed by asymptomatic hyperCKemia (24%; CK, creatine kinase), then foot drop (12%). 6% of patients presented with proximal arm weakness, 4% with axial weakness and 4% with facial diplegia. Median time from symptom onset to diagnosis was 9y. Median age at diagnosis was 70.5y. 16% of patients needed a walking aid. When tested, 86.5% of patients had impaired swallowing and 56% had elevated cytosolic nucleotidase-1A antibodies. Only 1/26 patients who received immunotherapy had minimal improvement. Upon follow-up, most patients had generalization of their weakness with a decline in their strength summated score of 0.082/mo. A significant proportion of IBM patients may have an atypical presentation. Recognition of such heterogeneity could improve early diagnosis, prevent unnecessary immunotherapy, and provide insight for future diagnostic criteria development and clinical trials.

Characteristics of clinical and laboratory indexes in patients with liver disease with positive anti-liver cytosol antibody

Objective: To analyze the characteristics of clinical and laboratory indexes in patients with liver disease with positive anti-liver cytosol antibody type 1 (anti-LC1), in order to provide references for clinical and differential diagnosis. Methods: The clinical data of 23 832 inpatients and outpatients with positive anti-LC1 autoantibodies detected in routine autoantibody test from January 2010 to January 2020 were retrospectively analyzed, and their clinical and laboratory indexes were compared. Western blotting was used to detect anti-LC1, anti-soluble liver antigen antibody (anti-SLA), anti-glycoprotein 210 antibodies and anti-nucleosome 100 antibodies. Indirect immunofluorescence assay was used to detect anti-nuclear antibody (ANA), anti-mitochondrial antibody, anti-Smooth muscle antibody (ASMA), anti-liver and kidney microsomal antibody (anti-LKM) and other autoantibodies. Normally distributed measurement data between the two groups were compared by independent-sample t-test, and the multiple groups comparison were compared by one-way analysis of variance. Non-normally distributed measurement data were compared by non-parametric rank sum test. Results: 38 anti-LC1 positive patients were detected in 23832 autoantibody tests. The age of initial diagnosis ranged from 11.0 to 84.0 (50.6 ± 16.0) years. There were 8 males (21.1%) and 30 females (78.9%). A total of 31 cases (81.6%) were positive for anti-LC1 and ANA, and the dominant karyotype was speckled pattern, accounting for 54.8%. Five cases (13.2%) were positive for ASMA, and no simultaneous positive with anti-LKM or anti-SLA. Among the 38 anti-LC1 positive patients, 9 were diagnosed with autoimmune hepatitis (AIH), 6 with possible AIH, 6 with primary biliary cholangitis (PBC), 8 with hepatitis B, 2 with hepatitis C, 1 with alcoholic liver disease, 2 with non-alcoholic fatty liver disease, 1 with drug-induced liver injury, 1 with hepatolenticular degeneration, and 2 with tumor. Confirmed and probable AIH cases accounted for 39.5% (15/38) of anti-LC1 positive cases. Among anti-LC1 positive patients, 47.4% (18/38) had entered the stage of liver cirrhosis. AIH group globulin level was higher than HBV group (P = 0.006) and other disease groups (P = 0.001). AIH group IgG level was higher than PBC group (P = 0.027), HBV group (P = 0.009) and other disease groups (P = 0.004). the of the PBC group IgM level was higher than AIH group (P = 0.003), HBV group (P = 0.003) and other disease groups (P = 0.006). Conclusion: Anti-LC1 is not only detected in AIH, but also observed in patients with primary biliary cholangitis, hepatitis B and C, alcoholic and non-alcoholic liver disease, drug-induced liver injury, hereditary metabolic liver disease and tumor. In addition, it is mainly female gender dominance and nearly half of ANA-positive young, middle-aged and elderly patients develop liver cirrhosis. For the diagnosis of type 2 autoimmune hepatitis, whether anti-LC1 is a specific antibody needs further research, but if AIH is highly suspected, this antibody can be used as a substitute.

E3 ubiquitin ligase TRIM21 restricts hepatitis B virus replication by targeting HBx for proteasomal degradation

As a cytosol ubiquitin ligase and antibody receptor, Tripartite motif-containing 21 (TRIM21) has been reported to mediate the restriction of hepatitis B virus (HBV) through an HBx-antibody-dependent intracellular neutralization (ADIN) mechanism. However, whether TRIM21 limits HBV replication by targeting viral proteins remains unclarified. In this study, we demonstrate that TRIM21 inhibits HBV gene transcription and replication in HBV plasmid transfected and HBV-infected hepatoma cells. RING and PRY-SPRY domains are involved in this activity. TRIM21 interacts with HBx protein and targets HBx for ubiquitination and proteasomal degradation, leading to impaired HBx-mediated degradation of structural maintenance of chromosomes 6 (Smc6) and suppression of HBV replication. TRIM21 fails to restrict the replication of an HBx-deficient HBV. And knock-down of Smc6 largely impairs the anti-HBV activity of TRIM21 in HepG2 cells. In a hydrodynamic injection (HDI)-based HBV mouse model, we confirm an in vivo anti-HBV and anti-HBx therapeutic effect of TRIM21 by over-expression or knocking-out strategy. Our findings reveal a novel mechanism that TRIM21 restricts HBV replication through targeting HBx-Smc5/6 pathway, which may have an implication in the future TRIM21-based therapeutic application.

TRIM21 inhibits porcine epidemic diarrhea virus proliferation by proteasomal degradation of the nucleocapsid protein.

Tripartite motif protein 21 (TRIM21) is an E3 ubiquitin ligase and cytosolic antibody receptor of the TRIM family. Previous reports have indicated that TRIM21 plays an important role during viral infection. This study aimed at examining the role of TRIM21 in the replication of porcine epidemic diarrhea virus (PEDV) and showed that TRIM21 inhibits PEDV proliferation by targeting and degrading the nucleocapsid (N) protein through the proteasomal pathway. Furthermore, the endogenous expression of TRIM21 was found to be downregulated by PEDV infection in Vero and LLC-PK1 cells. Overexpression of TRIM21 inhibited PEDV replication, whereas knockdown of TRIM21 increased viral titers and N protein levels. TRIM21 was found to interact and colocalize with the N protein, and the TRIM21-mediated antiviral effect was dependent on its ubiquitin ligase activity, which engages in polyubiquitination and degradation of the N protein in a proteasome-dependent manner. Taken together, these findings provide information about the role of TRIM21 in PEDV proliferation and increase our understanding of host-virus interactions.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00705-021-05080-4.

Target-induced clustering activates Trim-Away of pathogens and proteins.

Trim-Away is a recently developed technology that exploits off-the-shelf antibodies and the RING E3 ligase and cytosolic antibody receptor TRIM21 to carry out rapid protein depletion. How TRIM21 is catalytically activated upon target engagement, either during its normal immune function or when repurposed for targeted protein degradation, is unknown. Here we show that a mechanism of target-induced clustering triggers intermolecular dimerization of the RING domain to switch on the ubiquitination activity of TRIM21 and induce virus neutralization or drive Trim-Away. We harness this mechanism for selective degradation of disease-causing huntingtin protein containing long polyglutamine tracts and expand the Trim-Away toolbox with highly active TRIM21-nanobody chimeras that can also be controlled optogenetically. This work provides a mechanism for cellular activation of TRIM RING ligases and has implications for targeted protein degradation technologies.

Decompensated Cirrhosis as Presentation of LKM1/LC1 Positive Type 2 Autoimmune Hepatitis in Adulthood. A Rare Clinical Entity of Difficult Management

Background: Autoimmune hepatitis (AIH) is a chronic and aggressive liver disease that rapidly evolves into cirrhosis and end-stage liver disease if not timely diagnosed and treated with immunosuppressive therapy. AIH is classified into type 1 and type 2 according to the autoantibody pattern, with smooth muscle antibodies and/or antinuclear antibodies as serological markers of AIH-1, while antiliver cytosol antibody type 1 and/or antiliver/kidney microsomal antibody type 1 characterize type 2 AIH, which mainly affects children, including infants, and adolescents. Case Summary: We describe a case of type 2 AIH, clinically onset in a 34-year-old woman with decompensated cirrhosis. Only a thorough analysis of the autoantibody profile allowed for a diagnosis of an AIH-2 evolved into cirrhosis. The patient received a moderate corticosteroid therapy without achieving optimal disease control. We discuss the controversial decision of whether or not to treat the patient with immunosuppressive therapy, which should be balanced with the potential risk of infectious and other complications. A review of the literature on the management of patients with autoimmune cirrhosis is also presented. Conclusions: AIH-2 can be clinically onset in adult patients with cirrhosis and its complications, without being preceded by major clinical signs. Due to the difficult management of cirrhosis with immunosuppressive treatments, a patient-tailored strategy with a case-by-case approach is needed to prevent major complications such as infections, potentially precluding liver transplantation the only curative therapy.

Demonstration of intracellular trafficking, cytosolic bioavailability, and target manipulation of an antibody delivery platform

Intracellular antibody delivery into live cells has significant implications for research and therapeutic applications. However, many delivery systems lack potency due to low uptake and/or endosomal entrapment and understanding of intracellular delivery processes is lacking. Herein, we studied the cellular uptake, intracellular trafficking and targeting of antibodies using our previously developed Hex antibody nanocarrier. We demonstrated Hex-antibodies were internalized through multiple endocytic routes into lysosomes and provide evidence of endo/lysosomal disruption and Hex-antibody release to the cytosol. Cytosolic antibodies retained their bioactivity for at least 24 h. Functional effect of Hex delivered anti-STAT3 antibodies was evidenced by inhibition of nuclear translocation of cytosolic transcription factor STAT3. This study has generated understanding of key steps in the Hex intracellular antibody delivery system and will facilitate the development of effective cytosolic antibody delivery and applications in both the therapeutic and research domains.

HDAC6 regulates antibody-dependent intracellular neutralization of viruses via deacetylation of TRIM21

Tripartite motif-containing protein 21 (TRIM21) is a cytosolic antibody receptor that targets the internalized virus-antibody complex to the proteasome for degradation. However, the precise mechanism regulating TRIM21 activity is unknown. Here we show that TRIM21 is a substrate of histone deacetylase 6 (HDAC6) and that its function is regulated by acetylation. HDAC6 interacts with TRIM21 through its PRYSPRY motif and deacetylates TRIM21 at lysine 385 and lysine 387, thus promoting its homodimerization. Inhibiting HDAC6 activity increases TRIM21 acetylation, and hyperacetylation blocks TRIM21 dimerization and ubiquitination, preventing its binding to the virus-antibody complex and its degradation via the ubiquitin-proteasome pathway. HDAC6 depletion or inhibition increases virus accumulation in cells, indicative of an impaired capacity for antibody-dependent intracellular neutralization of viruses, whereas TRIM21 acetylation-deficient K385/387R mutant rescues HDAC6 depletion-caused ADIN impairment. These findings provide evidence for HDAC6 as a novel regulator of TRIM21-mediated intracellular innate immunity.

Intracellular neutralisation of rotavirus by VP6-specific IgG.

Rotavirus is a major cause of gastroenteritis in children, with infection typically inducing high levels of protective antibodies. Antibodies targeting the middle capsid protein VP6 are particularly abundant, and as VP6 is only exposed inside cells, neutralisation must be post-entry. However, while a system of poly immune globulin receptor (pIgR) transcytosis has been proposed for anti-VP6 IgAs, the mechanism by which VP6-specific IgG mediates protection remains less clear. We have developed an intracellular neutralisation assay to examine how antibodies neutralise rotavirus inside cells, enabling comparison between IgG and IgA isotypes. Unexpectedly we found that neutralisation by VP6-specific IgG was much more efficient than by VP6-specific IgA. This observation was highly dependent on the activity of the cytosolic antibody receptor TRIM21 and was confirmed using an in vivo model of murine rotavirus infection. Furthermore, mice deficient in only IgG and not other antibody isotypes had a serious deficit in intracellular antibody-mediated protection. The finding that VP6-specific IgG protect mice against rotavirus infection has important implications for rotavirus vaccination. Current assays determine protection in humans predominantly by measuring rotavirus-specific IgA titres. Measurements of VP6-specific IgG may add to existing mechanistic correlates of protection.

Assembly and Folding Properties of Cytosolic IgG Intrabodies

Intrabodies, antibodies expressed within cells, offer an interesting way to target intracellular molecules, making them potentially useful for biotechnology and medicine. However, it remains controversial whether full-size IgG intrabodies expressed in the reducing environment of the cytosol of mammalian cells are workable and structurally sound. Herein, we settle this issue with a systematic investigation of the structure and functionality of four chimeric IgG1s with distinct variable (V) domains but identical constant (C) domains. Full-size IgGs expressed in the cytosol of HEK293 cells were either assembly-competent or -incompetent, depending on the intrinsic properties of the V regions. Structural integrity of the C region is required for H:L association and the formation of a functional antigen-binding site. Partial intrachain disulfide bond formation occurs in both H and L chains of cytosolic IgG intrabodies, whereas interchain disulfide bond formation was absent and dispensable for functional assembly. IgG1s expressed in the cytosol and via the ER were shown to assemble differently. Our findings provide insight into the features and possible utilization of full-size IgGs as cytosolic antibodies in biotechnological and medical applications.